Good idea but beyond the scope of many enthusiasts' skills, and paying for that work will likely exceed the cost of the fork replacement. Is there some kind of "Problem Solvers" product out there that would work in this case? That company produced many little niche items to address similar problems. Shame, because the fork itself seems fine otherwise.

It's a shock sometimes when a new cyclist encounters a problem like this that seems like it should have a cheap and easy solution but the vicissitudes of the market and its products, the basic hardware and mechanics involved, and cost/benefit analyses radically influence the outcome as they do.

*edit after further thought*

How deep are the threads remaining in that cantilever boss? Can it be drilled and threaded further than it already is? I can see a way in which the existing broken-off boss might continue to be used if you could get a longer bolt in there to hold everything together, but this is the sort of thing I would do on my own bike and not necessarily a solution I would suggest a shop sell a customer. New fork would be safest. Also that is a Cro-Mo fork according to the sticker visible in the photo, so the OP should be aware that the replacement might not be as nice, or the shop could be selling him an even better one. Good luck!

Instead of replacing the fork they could always hang a sidepull or centerpull front brake on it from the hole in the crown, bypassing the need for cantis entirely.